Update proto alpha versions (#1556)

Author: trask

.github/renovate.json5

@@ -8,12 +8,13 @@
       "matchPackageNames": [
         "io.opentelemetry.instrumentation:opentelemetry-instrumentation-bom-alpha",
         "io.opentelemetry.semconv:opentelemetry-semconv",
-        "io.opentelemetry.semconv:opentelemetry-semconv-incubating"
+        "io.opentelemetry.semconv:opentelemetry-semconv-incubating",
+        "io.opentelemetry.proto:opentelemetry-proto"
       ],
       // Renovate's default behavior is only to update from unstable -> unstable if it's for the
       // major.minor.patch, under the assumption that you would want to update to the stable version
       // of that release instead of the unstable version for a future release
-      // (but there's never any stable version of these artifacts so this logic doesn't apply)
+      // (TODO remove once the artifacts above release stable versions)
       "ignoreUnstable": false
     },
     {